The invention relates to a method for coding data packets that include at least one control command and at least one data byte.
Contactless identification systems operate on the basis of contactless transmission technologies. These may be based on an electromagnetic method, e.g. using light or infrared, or on ultrasound, etc. Systems of this type are used, for example, to identify persons, or moving goods or means of transport. To this end, the necessary data are transmitted from a transceiver device via a contactless data transmission path, e.g. via an air interface, to a data medium and back. The contactless identification system also permits the recording of data, e.g. while the data medium moves past, without the latter having to be inserted into or drawn through a read/write device. Data media of this type are used, for example, as travel tickets with an electronically reloadable credit, whereby the appropriate travel fare is automatically deducted when the means of transport is used.
So that the data media can be used for unlimited periods of time, the integration of chemical energy stores, e.g. batteries, is preferably dispensed with. The requisite electrical energy of the data media is therefore drawn in a contactless manner externally, i.e. from an energy source originating from the transceiver device, e.g. an electrical or magnetic field. Suitable transmission and coding methods are therefore required for communication between the transceiver device and data media of this type. On the one hand, only specific frequency bands are normally released for the transmission of data, e.g. the ISM frequency bands (Industrial, Scientific & Medical), for industrial, scientific and medical applications. Possible national radio regulations may define, inter alia, the modulation bandwidths and field strengths which must be maintained. On the other hand, the transmission and coding methods must also ensure the energy supply of the electronics on the data medium.
Such methods are known, e.g. according to the standard ISO/IEC 15693, Part 2, “Air Interface and Initialization”. Methods of this type enable continuous energy supply of the data medium electronics, originating from the energy of the applied carrier frequency of the transceiver device. Here, the carrier frequency is de-activated for a maximum time interval only in order to modulate the data which are to be transmitted. Within this time interval, an energy store previously charged by the electrical or magnetic field must be able to bypass the energy supply of the data medium electronics. A resonant circuit or capacitor on the data medium, for example, can be used as a temporary energy store. Here, the data are coded by de-activating the carrier at defined positions within a cyclical time pattern. Taking into account the aforementioned maximum time interval, the standard furthermore defines the field strength limit values for the sidebands produced through modulation at a specific carrier frequency. The time ratio of the activated to the de-activated carrier frequency on the one hand determines the level of the sideband modulation. Furthermore, consecutive changeovers from the activated to the de-activated carrier frequency also contribute to a significant increase in the sideband modulation. The need to adhere to the sideband limits defined in the standard results in a maximum possible data rate. For an ISM frequency band according to standard 15693, Part 2, page 6, for example, this data rate is 26.48 kbit/s.
The disadvantage of the aforementioned method according to the standard ISO/IEC 15693, Part 2, is that the data rate which is possible for a frequency band is no longer adequate for many applications, or insufficient time is available to transmit the data.